In the interests of the environment, and to comply with increasingly stringent regulatory demands, it is necessary to increase the amount of biofuels used in automotive fuels.
Biofuels are combustible fuels, typically derived from biological sources, which result in a reduction in “well-to-wheels” (ie from source to combustion) greenhouse gas emissions. In diesel fuels for use in compression ignition engines, the most common biofuels are fatty acid alkyl esters (FAAEs), in particular fatty acid methyl esters (FAMEs) such as rapeseed methyl ester and palm oil methyl ester; these are used in blends with conventional diesel fuel components.
There can however be drawbacks associated with the use of FAMEs in diesel fuels, in particular at higher concentrations. The addition of a FAME to a diesel fuel raises its cloud point, to an extent dependent on the FAME concentration, and also raises the cold filter plugging point (CFPP) of the fuel This can compromise the performance of the fuel, both during handling in the fuel distribution system and on-board vehicles.
FAMEs and their oxidation products also tend to accumulate in engine oil, leading to changes in the properties of the lubricant and a number of related issues. FAMEs have also been associated with deposit-related and oxidation stability issues. This has limited their use in modern FAME/diesel blends, typically to below around 10% v/v. At higher concentrations FAMEs can also cause fouling of fuel injectors.
Moreover, due to the incomplete esterification of oils (triglycerides) during their manufacture, FAMEs can contain trace amounts of glycerides, in particular monoglycerides. These glycerides tend, on cooling, to crystallise out before the FAMEs themselves, and can cause fuel filter blockages. This too can compromise the cold weather operability of a FAME-containing diesel fuel.
The most common monoglycerides present in FAMEs are the saturated C16:0 (palmitic) and C18:0 (stearic) monoglycerides, and the unsaturated C18:1 (oleic) and C18:2 (linoleic) monoglycerides. The amount of each of these which is present in a FAME will depend on the nature of the FAME and also on the process by which it was manufactured. It is the saturated monoglycerides which appear to have the most detrimental effect on cold weather performance of FAME-containing fuels, since they are less soluble than for instance triglycerides and more prone to precipitate at low temperatures; they are also typically present at higher levels than triglycerides (the European specification EN 14214:2003 for FAMEs for use as diesel fuels allows 0.8% w/w of monoglycerides but only 0.2% w/w of triglycerides). Certain monoglycerides are also thought to be responsible for corrosion and injector fouling issues in fuels containing FAMEs.
These factors together mean that it can be difficult to formulate diesel fuel/FAME blends within the relevant regulatory specifications, particularly in colder climates where specifications require maximum cloud points and CFPPs to be lower than in more temperate regions. As a result, FAMEs are typically included in diesel fuels, in particular winter grade fuels, at relatively low concentrations. Moreover FAMEs for use in diesel fuels need to be prepared to relatively stringent specifications as regards their glyceride contents, thus increasing the cost of their production. FAMEs for use in current diesel fuels are typically required to contain a maximum of 0.8% w/w monoglycerides (EN 14214).
In addition to fuel formulation constraints, their poor low temperature behaviour can make the neat FAMEs themselves difficult to handle and to transport in colder climates. In particularly severe conditions, heated supply lines may be needed in order to transport and handle the materials, and in some situations this makes certain FAMEs impractical, or at least unattractive, to work with at all.
It would be desirable to provide new biofuel-containing diesel fuel formulations which could overcome or at least mitigate the above problems.
A number of alternative biofuel components have accordingly been investigated, amongst which are the glycerol ethers. Since glycerol is a byproduct of the transesterification of oils to produce FAMEs, it is available in increasing abundance as the demand for higher biofuel contents increases. It can accordingly provide a commercially attractive starting material from which to generate further biofuels: it can for example be reacted with methanol or ethanol to yield, respectively, methoxy- or ethoxypropane ethers.
Several published documents describe the synthesis of glycerol ethers and their use in diesel fuel formulations, for example US-A-2008/0293602 (glycerol monoethers, and also 1,2,3-trimethoxypropane); US-A-2008/0167503 (tertiary alkoxypropane ethers such as those formed from t-butanol); and WO-A-2005/093015 (mixtures of mono-, di- and trialkoxy ethers of glycerol, in particular t-butyl ethers).
Glycerol ethers tend to have better low temperature properties than FAMEs (for example lower cloud points, pour points and CFPPs). U.S. Pat. No. 6,015,440 teaches that they can be used to reduce the viscosities and the cloud points of diesel fuels. WO-A-2008/112910 states that they can improve the viscosity and the low temperature properties of biodiesel fuel components such as FAMEs, implying a preference for C4-C5 alkoxy ethers such as those formed from t-butanol or isobutanol.
Other documents have suggested the use of glycerol ethers in diesel fuels as lubricants (US-A-2008/0293602), as cetane improvers and to reduce emissions. GB-A-2 368 594 for example discloses the use of 50% w/w or more of trialkoxyalkanes—including triethoxyalkanes and 1,1,3-trialkoxyalkanes—to reduce soot (particulate) emissions from diesel fuels. WO-A-98/56879 discloses the use of relatively low levels of an alkoxyalkane, in particular a 1,1,3-trialkoxypropane such as 1,1,3-triethoxypropane, to increase the cetane number of a diesel fuel. WO-A-2007/061903 suggests that partially etherified glycerol derivatives, in combination with alcohols, can lower NOx and particulate emissions from a diesel fuel, as well as improving its cetane value and lowering its viscosity.
U.S. Pat. No. 5,308,365 describes the use of di- and trialkoxypropane ethers in diesel fuels in order to reduce particulate emissions. A preference is expressed for C4-C5 alkoxy ethers, in particular t-butyloxy ethers. The trialkoxy ethers mentioned include 1,2,3-trialkoxy ethers.
WO-A-2009/141564 describes a process for the simultaneous production of FAAEs and glycerol ethers, using ethanol to convert the glycerol byproduct of the FAAE production into a mixture of ethoxy ethers. The document makes clear that the mono- and di-ethoxy ethers are the preferred products, and the process is ideally allowed to continue only until a suitable mixture of those products has been formed. This mixture, together with the FAAE generated at the same time, may then be incorporated into a diesel fuel.